1. Field of the Invention
The present invention relates generally to refractory materials and articles, and their methods of manufacture, and more particularly to refractory materials and articles with high resistance to molten metals, molten salts, and high temperature corrosive environments.
2. Description of Related Art
In many fields there is a need for highly refractory, chemically resistant, and thermodynamically stable materials and articles. Materials and articles with high melting points and with good mechanical and electrical properties are needed for use in thermochemical processing technologies incorporating materials in the form of molten metals, molten salts, reactive gases, and other corrosive chemicals. One particularly important area of application is crucibles, molds, and other containment vessels. Another important area is processing tools like stirring rods, transport tubes, temperature probes, bricks and mortar, and coated structures.
Materials and articles are required which can withstand high operating temperatures and can be exposed to a variety of difficult to handle materials, ranging from reactive metals like Ti, NiTi, Ti alloys, nickel and cobalt based super alloys and stainless steels, Zr, Hf, V, Nb, Ta, and Be, to lanthanide metals La to Lu, to actinide metals like U and Pu, as well as commonly used aggressive salts like alkali-halides (e.g., NaCl, KCl, LiCl) and alkaline earth-halides (e.g., CaCl2, CaF2). In addition, refractory materials and articles must withstand thermal cycling conditions, i.e., heating and cooling, without spalling or cracking; and must be able to be tailored to be good thermal and electrical conductors or thermal and electrical insulators.
Some of the problems associated with thermochemical processes are the result of high temperatures and very chemically reactive materials used for specific functions in the processes. Molten salts and metals are extremely aggressive at high temperatures and attack the molds or crucibles used to contain them in the processing vessel.
Metals like tungsten and tantalum have been used for crucibles, but the molten salts gradually erode them and molten metals can form alloys or intermetallic compounds with them. When the metals are corroded or no longer useable, they are discarded as waste. Magnesia (MgO), calcia, (CaO), tetragonally stabilized zirconia (ZrO2), and yttria (Y2O3) are examples of ceramic materials that have been used for crucibles, but these materials are either wetted or attacked by the melts, or crack easily from thermal shock, or from stresses that build up at the interface between the melt and crucible, so they get limited use and are then discarded as waste.
Some coating technologies have yielded promising results, although the materials and articles produced by these methods have inherent problems as well. Important examples of inherent problems are those associated with protective coatings that are either dip-coated or plasma sprayed on crucibles and molds. These coatings may have different thermal and chemical properties than their host; thus, property mismatches cause spalling or cracking of the protective coating, causing contamination of the melt, and allowing the host material to be exposed to the environment and attacked.
In the past, contractors to the US Department of Energy have utilized crucibles and molds containing beryllia (BeO), zirconia (ZrO2), and yttria (Y2O3) for containing reactive metals like beryllium and uranium. These materials crack, spall, or react into the melts, causing premature failure of the refractory and add impurities to the final cast-metal products. Other technologies widely employed for processing reactive metals are arc-melting, electron-beam melting, and cold-wall induction melting. Under these conditions, the reactive molten metals are frozen, or solidified, quickly at the surface of the containment vessel, and thus are not allowed time to react with the container. However, these processes require large capital investments, are complicated, and have high operating costs. Further, an inherent problem with these technologies is that they do not allow for thorough mixing (or alloying) which may be desired or required for the final properties of the products.
U.S. Pat. No. 5,084,312 to Krikorian et al describes molten metal containment vessels with rare earth or rare earth like sulfide and oxysulfide coatings which inhibit wetting. Also described therein are conventional materials for containment vessels, including graphite, refractory metals, oxides, and fluorides. U.S. Pat. No. 4,363,995 to Crawford et al describes metal oxide or metal sulfide coatings. U.S. Pat. No. 4,876,725 to Furukawa et al describes high density sintered articles of silicon carbide. U.S. Pat. No. 3,890,140 to Asbury describes an aluminum titanate crucible for molten uranium. U.S. Pat. No. 5,934,900 to Billings describes numerous ceramic materials and ceramic-metal articles for containing a variety of molten materials and this application relates to and expands on some of those materials and articles.
Accordingly, it is an object of the invention to provide improved refractory materials and articles with high resistance to molten metals, molten salts, and high temperature corrosive environments, and materials that can be tailored to have desirable thermal and electrical properties for specific process requirements.
The invention is a class of refractory ceramic and composite materials that are used to form refractory articles, or as coatings for refractory articles, comprising nitrides, mixed nitrides and oxides, and oxynitrides of Group IV metals selected from Hf, Zr, and Ti; and nitrides, mixed nitrides and oxides, and oxynitrides of Group III metals selected from Sc, Y, and the lanthanides La through Lu, alone or combined or reacted with the above Group IV (Hf, Zr, Ti) refractory materials. Thus the refractory materials of the invention encompass nitrides, nitride/oxide mixtures, and oxynitrides of the Group IV (Hf, Zr, Ti) metals, and Group III (Sc, Y, and the lanthanides La through Lu) metals, and mixtures and composites thereof. The invention encompasses both stoichiometric and sub-stoichiometric materials. Further, the invention includes the refractory articles formed of, or coated with, these refractory materials.
In particular the invention is directed to articles comprising containment vessels, particularly crucibles and molds, and also spray nozzles. Also in particular the invention is directed to articles made of the nitrides of hafnium (Hf), including pure nitrides, mixed nitrides and oxides, and oxynitrides. The Hf nitride materials may also be combined with or reacted with the nitrides of the Group III metals (Sc, Y, and the lanthanides La through Lu), including pure nitrides, mixed nitrides and oxides, and oxynitrides. The articles may be made entirely of these materials, or these materials may form an external coating on an internal body formed of metal, ceramic, or graphite.